Apparatus and process for coating



Jan. 30, 1962 A. G. WALSH 3,019,129 APPARATUS AND PROCESS FOR COATING Filed Aug. 10, 1959 2 Sheets-Sheet 1 o ooo zoookooooog 2/ Ii FIGURE FIGURE 2.

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APPARATUS AND PROCESS FOR COATING Filed Aug. 10. 1959 21 Sheets-Sheet 2 United States Patent Ofifice 3,l9,129 Patented Jan. 30, 1962 APPARATUS AND PROCESS FOR COATING Arthur G. Walsh, Essex Fells, N.J., assignor to National Research Corporation, Cambridge, Mass, a corporation of Massachusetts Filed Aug. 10, 1959, Ser. No. 832,705 11 Claims. c1.117-101 This invention relates to coating and more particularly to vacuum deposition processes and apparatus wherein a material is evaporated in a vacuum and condensed on a substrate.

It is a principal object of this invention to provide new and improved means for the deposition of a material such as aluminum to form a coating of a desired thickness on a substrate such as flexible sheet material, wire, thread, or discrete objects.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the apparatus possessing the features, properties and the relation of components and the process involving the several steps and the relation and order of one or more of such steps with respect to each of the others which is exemplified in the following detailed disclosure, and the scope of the application which will be defined in the claims.

For a more complete understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein:

FIG. 1 is a sectional schematic view of one preferred embodiment of the invention;

FIG. 2 is an end sectional schematic view taken on the line AA of FIG. 1; and,

FIG. 3 is a sectional schematic view of an alternate form of apparatus for carrying out the invention.

Without intent to limit the invention, it will initially be described in connection with the coating of a substrate by aluminum.

Briefly stated, the invention comprises a vacuum coating apparatus and method which includes an aluminum vapor source which will discharge aluminum vapors inwardly from a circumferential molten surface, thereby completely coating all surfaces of a substrate positioned near the axis of the circumference. In a preferred embodiment of the invention, the aluminum to be evaporated is supported on a moveable support and the support is rotated about an axis so that the molten aluminum will be held on the support by centrifugal force. The support is preferably cylindrical and positioned within a heating means such as an induction heating coil, so that the aluminum metal may be readily heated and maintained at vaporization temperature. The cylindrical support may be positioned horizontally or vertically depending on the substrate to be coated.

Referring now to FIGS. 1 and 2, wherein like numbers refer to like elements, there embodiment of the invention which is particularly adapted to the coating of strip or sheet material. In this embodiment, represents a vacuum-tight housing defining a vacuum chamber 12 arranged to be evacuated by a vacuum pumping system, not shown. The support for the aluminum to be vaporized is generally represented at 14 as a cylindrical member, the cylindrical support comprising an outer mechanical support 16 having an inner lining of a refractory material 18 which supports the molten aluminum 20. Drive and guide rolls 22 rotate the cylindrical support at a suificiently fast speed so that the molten aluminum is held on the whole interior surface of the cylinder, by centrifugal force. Refractory end blocks 24 and heat baffles 26 are preferably provided to limit escape of heat and aluminum vapors from 'the volume defined by the cylindrical support. The aluminum is heated to vaporization temperature by induction heating coils 28. Molten or solid aluminum is introduced by feeder means 30.

A suitable inlet compartment 32 and an exit compartment 34 are associated with the main vacuum chamber Iii to provide passage from the atmosphere into the high vacuum in the coating chamber and back out to the atmosphere again. These chambers may be of the type shown in Stoll Patent 2,384,500, for example. When the substrate 36 is a steel strip, it may be subjected to preheating as described in copending application Serial No. 729,292, filed April 18, 1958, and now US. Patent No. 2,906,641.

The vapor source 14 in one preferred embodiment is constructed by providing an outer shell 16 of tantalum or other refractory material of sufiicient strength to resist the centrifugal forces involved. This outer shell 16 is lined at 18 with a refractory material such as carbon or graphite. Liner 18 preferably has an inner surface of a carbide of one of the group IVa or Va metals as described in Patent 2,665,223. Such a surface is capable of resisting attack by molten aluminum for an extended period of time. The coating chamber may be provided with threading means and removable baffles for initially feeding the substrate through the interior of the molten aluminum source. Equally, it can be provided with temperature measuring mechanisms, coating thickness indicating means and the like, which are standard practice in the vacuum coating art.

The substrate may be wire, thread, metals or nonmetals. The aluminum film may be extremely thin (on the order of a few millionths of an inch), or it may be relatively thick (on the order of a few thousandths of an inch), depending upon the use to which the substrate is to be put.

Referring now to FIG. 3 wherein like numbers refer to like elements, there is shown an alternate embodiment of the invention which is particularly adapted to the coating of discrete objects, such as powders, nuts and bolts, and the like.

In carrying out the process of this embodiment, fine powder, for example, is placed in feed tank 40 having a vacuum lock, not shown. When the feed tank 40 and the vacuum chamber 12 have been evacuated by suitable vacuum pumping systems, not shown, valve: 42 is opened allowing the powder into the vacuum vibratory feeder 44. The vibratory feeder is preferably equipped with means for heating or cooling the substrate prior to coating. The vibrating of the feeder will cause the powder collected thereon to travel to the end thereof and then fall by gravity through the cylindrical aluminum vapor source 14a.

In this case, the axis of the cylindrical vapor source is mounted in a vertical position so that free falling objects can pass through the vapor source in a path generally parallel to the axis. In other respects, the construction of the source can be substantially identical to that illustrated in FIGS. 1 and 2. A cooling mechanism shown as a cone 48 is provided below the source to catch the free falling objects which can be removed through a valved connection, indicated generally at 50, into a removable product hopper 52. If desired, a product elevator can be positioned in the main vacuum tank a for recycling the product, in the event that one passage of the product through the cylindrical vapor source 14a does not provide adequate coating.

While a free flowing powder has been illustrated as falling through the vertically mounted aluminum source 14a, numerous other arrangements may be provided. For example, cooled baffles may be positioned along the path of travel of the powder to partially cool the powder during coating and to cause it to tumble so as to assure even coating on all sides of the powder. When larger particles, such as nuts and bolts and other fabricated pieces are to be coated, they can be passed slowly through the axis of the coater on jigs, chains and other suitable fixtures in those cases where free falling of the objects does not give sufficient residence time to provide an adequately thick coating.

While preferred embodiments of the invention have been described above, numerous modifications thereof can be employed without departing from the scope of the invention. For example, numerous types of substrates can be coated such as fibers, nonmetals, organic materials, solid propellants, catalyst supports and numerous other subdivided materials. Equally, the particulate materials, passed through the coating chamber can be metallic objects such as steel, magnesium nuts and bolts and other objects onto which a decorative or corrosion-resistant coating is to be applied.

Additionally, while the cylindrical support 16 is preferably constructed of a tantalum, it can, however, be any suitable heat resistant metal alloy or other material. Similarly, in place of graphite or carbon liner 18, boron nitride or the carbides, nitrides and borides of refractory metals are suitable for practicing the invention. The principal requirements being that the materials be capable of withstanding the elevated temperatures required for aluminum vaporization and have a vapor pressure of less than 0.1 micron Hg abs. at 1500 C.

Two preferred embodiments of the invention have been described above, one in which a horizontal cylinder is provided, and the other in which a vertical cylinder is provided. These cylinders can be replaced by individual crucibles traveling in a cylindrical path, or series of rings or numerous other geometrical configurations. The important aspect of the invention is that the molten aluminum surface is held at an angle to the horizontal by centrifugal force, thereby providing direction to the coating vapors other than upwards and also providing coating on all sides of a substrate positioned adjacent to the surface of revolution defined by the rotating molten aluminum surface. Equally the surface of revolution of the molten metal can be parabolic rather than cylindrical.

Since certain changes may be made in the above process and apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Apparatus for coating a substrate with aluminum wherein aluminum is vaporized in vacuum and condensed on said substrate, said apparatus comprising a vacuum chamber, means for evacuating said chamber, means for introducing said substrate into said chamber, means providing a source of aluminum vapors in said chamber, said means comprising a support for molten aluminum, means for rotating said support around an axis to hold the surface of the molten aluminum at an angle to the horizontal by centrifugal force, means for heating said molten aluminum to a sufficiently elevated temperature to vaporize the aluminum, and means for positioning the substrate within the path of aluminum vapors traveling normally from the surface of revolution defined by the rotatin molten aluminum.

2. The apparatus of claim 1 wherein said support comprises a plurality of individual crucibles.

3. The apparatus of claim 1 wherein said support comprises a plurality of separate ring members.

4. Apparatus for coating a substrate with aluminum wherein aluminum is vaporized in vacuum and condensed on said substrate, said apparatus comprising a vacuum chamber, means for evacuating said chamber, means for introducing said substrate into said chamber, means providing a source of aluminum vapors within said chamber, said source means comprising a cylindrical support having an inner surface for holding molten aluminum, means for rotating said support around its axis to hold the molten aluminum on the inner surface of said support by centrifugal force, means for heating said molten aluminum to vaporization temperatu e, means for moving the substrate to be coated through said aluminum source generally parallel to the axis of rotation and within the cylinder of revolution defined by the rotating molten aluminum surface.

5. Apparatus for coating a plurality of discrete objects wherein aluminum is vaporized in vacuum and condensed on said discrete objects, said apparatus comprising a vacuum chamber, means for evacuating said chamber, means of providing a source of aluminum vapors in said chamber, said source means comprising a cylindrical support having its axis vertically positioned in said chamber and having an inner surface for supporting molten aluminum, means for rotating said support around said axis to hold the molten aluminum on the surface of said support by centrifugal force, and means for introducing said discrete objects into said chamber whereby said discrete objects fall by gravity through said cylinder of revolution.

6. The apparatus of claim 5 wherein said discrete objects comprise metal objects.

7. The apparatus of claim 5 wherein said discrete objects comprise powders.

8. Apparatus for coating a sheet material with aluminum wherein aluminum is vaporized in vacuum and condensed on said sheet material, said apparatus comprising a vacuum chamber, means for introducing said sheet material into said chamber, means providing a source of aluminum vapors within said chamber, said source means comprising a cylindrical support having its axis horizontally positioned in said chamber and havmg an inner surface for supporting molten aluminum, means for rotating said support around its axis to hold the molten aluminum to the surface of said support by centrifugal force, means for heating said molten aluminum to vaporization temperature, and means for moving the substrate to be coated through said aluminum source generally parallel to the axis of rotation and within the cylinder of revolution defined by the rotating molten aluminum surface.

9. The apparatus of claim 8 wherein said sheet material comprises a plurality of fibers.

10. In the process for coating a substrate with aluminum by vacuum-evaporating said aluminum and condensing said aluminum on said substrate, the improvement which comprises the steps of supporting molten aluminum on a cylindrical support positioned in said chamber while rotating said support around an axis to hold the molten aluminum on the inner surface of said support by centrifugal force, heating said aluminum thereon to a temperature sufficiently high to evaporate the aluminum and moving the substrate within the surface of revolution defined by the rotating molten aluminum surface to expose said substrate to the aluminum vapors generated from said surface and to deposit a coat of aluminum on the substrate.

11. In an apparatus for coating a substrate by a vapor deposition process of the type wherein a metal is vaporized under a high vacuum and vapors of the metal are condensed on the substrate in the vacuum chamber, the improvement which comprises a cylindrical support positioned in said chamber and having an inner surface for supporting molten metal, means for rotating said support around its axis to hold the molten metal on the inner surface of said support by centrifugal force, means for heating said molten metal to vaporization temperature and means for moving the substrate to be coated 10 References Cited in the file of this patent UNITED STATES PATENTS 2,746,831 Chapman May 22, 1956 FOREIGN PATENTS 760,543 Germany June 8, 1953 1,068,587 France Feb. 3, 1954 

8. APPARATUS FOR COATING A SHEET MATERIAL WITH ALUMINUM WHEREIN ALUMINUM IS VAPORIZED IN VACUUM AND CONDENSED ON SAID SHEET MATERIAL, SAID APPARATUS COMPRISING A VACUUM CHAMBER, MEANS FOR INTRODUCING SAID SHEET MATERIAL INTO SAID CHAMBER, MEANS PROVIDING A SOURCE OF ALUMINUM VAPORS WITHIN SAID CHAMBER, SAID SOURCE MEANS COMPRISING A CYLINDRICAL SUPPORT HAVING ITS AXIS HORIZONTALLY POSITIONED IN SAID CHAMBER AND HAVING AN INNER SURFACE FOR SUPPORTING MOLTEN ALUMINUM, MEANS FOR ROTATING SAID SUPPORT AROUND ITS AXIS TO HOLD THE MOLTEN ALUMINUM TO THE SURFACE OF SAID SUPPORT BY CENTRIFUGAL FORCE, MEANS FOR HEATING SAID MOLTEN ALUMINUM TO VAPORIZATION TEMPERATURE, AND MEANS FOR MOVING THE SUBSTRATE TO BE COATED THROUGH SAID ALUMINUM SOURCE GENERALLY PARALLEL TO THE AXIS OF ROTATION AND DWITHIN THE CYLINDER OR REVOLUTION DEFINED BY THE ROTATING MOLTEN ALUMINUM SURFACE. 